In large bowl, with mixer at high speed, beat egg whites and cream of tartar until soft peaks form; beat in vanilla extract. Beating at high speed, sprinkle in 1 1/4 cups confectioners' sugar, 2 tablespoons at a time, beating until sugar dissolves and whites stand in stiff peaks.

With rubber spatula or wire whisk, fold flour mixture into beaten egg whites just until blended. Then gently fold in hazelnut mixture. Do not overmix.

Spoon batter into ungreased 10-inch tube pan. Bake 35 minutes or until cake springs back when lightly touched. Invert cake in pan on funnel or bottle; cool completely in pan.

To serve, remove cake from pan. Serve with lemon sorbet. Garnish servings with hazelnuts and lemon leaves if you like.